GPU final render is very dark (depending on number samples)
Ok, so I found (thanks to Mr Zak!) that one of the differences between preview and final render was a Motion Blur. Even though it is disabled in preview and I had it disabled in render settings, when I enabled it in render panel and everything works again!

Though I definately remember it working from default blender file (where I suppose it was disabled?)... and this is seems like a bug, but hey, we found a workaround!